5 Answers
5

The oldest known source, but questionable explanation for the expression "baker's dozen" dates to the 13th century [...]. Bakers who were found to have shortchanged customers (some variations say that they would sell hollow bread) could be subject to severe punishment. To guard against the punishment of losing a hand to an axe, a baker would give 13 for the price of 12, to be certain of not being known as a cheat. [...]

It's not just bakers, and it's not just a dozen. See Wikipedia on lagniappe, which also has a fascinating etymology. I don't think the custom needs an elaborate explanation.
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Peter Shor Apr 21 '12 at 11:00

3

If you are talking about the 13th century or even the 16th then the packing of items on a tray or sheet is an anachronism. In these time and much later bread was baked on the oven floor and set with a peel. I doubt if the technology for steel trays was developed until the 18 century at the earliest.
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PurplePilotApr 21 '12 at 12:44

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Like @PurplePilot said, the geometrical explanation is stuff and nonsense: not all baked items are round, and they didn't use rectangular baking pans until many centuries after the term 'baker's dozen' was first used.
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MarthaªApr 30 '12 at 15:19

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And in fact the balderdash explanation has since been removed from Wikipedia, leaving just the accepted (although also doubtful) shortchanging explanation.
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MarthaªApr 30 '12 at 16:14

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@Malvolio: the whole "they added an extra loaf as insurance against getting their hand cut off" premise sets off all my urban legend detectors, because it sounds like typical middle-ages-bashing ("Look how cruel and nasty they were! And they used a lot of spices to hide the taste of spoiled meat!" [1. Spices were several order of magnitude more expensive than meat, and 2. Spoiled meat would have made them just as physically ill as it would make us, no matter how the well the taste was disguised.])
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MarthaªMay 7 '12 at 14:48

The Worshipful Company of Bakers is one
of the Livery Companies of the City of London. The Bakers' Guild is
known to have existed in the twelfth century. From the Corporation of
London, the Guild received the power to enforce regulations for
baking, known as the Assize of Bread and Ale. The violations included
selling short-weight bread and the addition of sand instead of flour.
(So that they could avoid punishment for inadvertently selling a
short-weight bread, bakers added a thirteenth loaf to a dozen, giving
rise to the term baker's dozen.) The Bread Assize remained in force
until 1863, when Parliament repealed it.

In earlier times when a heavy penalty was inflicted for short weight,
bakers used to give a surplus number of loaves, called the in-bread,
to avoid all risk of incurring a fine. The 13th was the vantage loaf.

Oh, I like that too: The "vantage loaf". It reminds me of Yahrzeit (Yarhzeit? no, Yahrzeit, I think) candles, though tangentially. They must have sufficient wax to burn continuously for a full 24 hours. I've noticed that they usually are good for at least 26, even 30 hours though, as I have used them during power outages, not just for commemoration of the dead.
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Ellie KesselmanApr 21 '12 at 11:41

I understand this differently. From what I was taught, the baker's dozen resulted from compassion.

The extra item was added by the baker so the households slave/servant would be able to consume one of whatever was being purchased, while walking home with a bag full of food for their master/employer.